The Collaborative International Dictionary of English v.0.48:

Noctambulist \Noc*tam"bu*list\, n.
   A somnambulist; a sleepwalker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

noctambulist
    n 1: someone who walks about in their sleep [syn: sleepwalker,
         somnambulist, noctambulist]